This one's about as reliable as dear ol' Mom always leaving a light on for you: Whenever Ma's big day rolls around, you can expect hundreds of those paid to know better to refer to it erroneously as Mothers Day or Mothers' Day. Follow this Google News link to see.
For the record, it's Mother's Day.
Mr. Rewrite isn't claiming this one is easy. Veterans/Veterans' Day, Presidents/Presidents' Day, Valentine's Day, St. Patrick's Day -- it's enough to drive someone batty. But those paid to know better should be able to keep them straight.
Having posted on this for years now, Mr. Rewrite knows the explanation by heart: Mother's Day was named to honor your mother. Why did someone decide this? Mr. Rewrite doesn't know and doesn't particularly care. He just wants someone to make the call so he can follow it.
Our friends across the pond have it easier. They just call it Mothering Sunday, removing the apostrophe angst.